This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The membrane of a human red blood cell (RBC) membrane is a composite structure that provides the principal control of both the cell's morphology and mechanics. It consists of a fluid lipid bilayer tethered to a 2D spectrin network, which determines the composite membrane's elasticity. The dynamic properties of this structure influence the ability of RBCs to transport oxygen in circulation. Current techniques for assessing the viscoelasticity of RBCs rely on applying external loads and are limited in obtaining dynamic information over a wide range of frequencies. We apply a non-contact optical interferometric technique to quantify the thermal fluctuations of RBC membranes over a broad range of spatial and temporal frequencies.